The populist Five Star Movement, which has led calls for immediate elections, said it would boycott Wednesday's vote because the new government would have no legitimacy.

"This government is not even worthy of a vote against it," said Giulia Grillo, head of the Five Star group in the Senate.

All major parties have called for election as soon as possible.

But before any vote can take place, Mattarella has said Italy needs a new electoral law to replace one that applies only to the lower house and could be declared illegitimate in January by the Constitutional Court.

Elections are not due until 2018 but could be called as soon as parliament finishes rewriting the electoral law. Gentiloni said he would "facilitate, if possible, the parliamentary forces' task of quickly defining new electoral rules".